EP2912890B1

EP2912890B1 - Supporting a base station to enter and leave sleep mode in a wireless communication system

Info

Publication number: EP2912890B1
Application number: EP13850895.7A
Authority: EP
Inventors: Huaming Wu
Original assignee: ZTE USA Inc
Current assignee: ZTE USA Inc
Priority date: 2012-10-29
Filing date: 2013-10-29
Publication date: 2020-01-08
Anticipated expiration: 2033-10-29
Also published as: EP2912890A4; US9351216B2; WO2014070785A1; CN104885520A; EP2912890A1; HK1214461A1; US20150245270A1; CN104885520B

Description

TECHNICAL FIELD

The present invention generally relates to wireless communication, and in particular, to methods of supporting a base station to enter and leave sleep mode when the base station's traffic load satisfies a predefined condition.

BACKGROUND

Power consumption of a wireless communication system is increasingly a concern as people use more and more mobile communication devices such as cell phones and tablets. A large number of base stations have been deployed to support the increasing number of mobile communication devices. Those base stations contribute a major portion of the power consumption of a wireless communication system. But little effort has been made to reduce the power consumption of base stations without adversely affecting the performance of the wireless communication system.
WO 2012/116467 A1 discloses a method and device for status management of base stations in a communication network, wherein overall traffic load of the base stations is measured and at least one of the base stations is transferred from an awaken mode to a sleep mode based on the measurement, thereby saving power of base stations. WO 2011/137345 is related to systems and methods for reducing interference and saving power for home Node Bs. The home Node Bs can be configured to remain in sleep mode until needed based on various criteria.

SUMMARY

The invention is set out in the appended set of claims.

BRIEF DESCRIPTION OF THE DRAWINGS

Different aspects of the present invention as well as features and advantages thereof will be more clearly understood hereinafter because of a detailed description of embodiments of the present invention when taken in conjunction with the accompanying drawings, which are not necessarily drawn to scale. Like reference numerals refer to corresponding parts throughout the several views of the drawings.

FIG. 1A depicts the deployment of multiple base stations with different types of coverage in accordance with some embodiments of the present application.
FIG. 1B depicts the communication interfaces between multiple base stations and mobile management entity (MME) servers in accordance with some embodiments of the present application.
FIGS. 2A and 2B are flow charts illustrating how an operation mode controller coordinates the operation of multiple base stations in accordance with some embodiments of the present application.
FIGS. 2C-2E are block diagrams illustrating different messages exchanged between the operation mode controller and the base stations in accordance with some embodiments of the present application.
FIG. 3A is a flow chart illustrating how one base station coordinates its operation with multiple base stations in accordance with some embodiments of the present application.
FIGS. 3B and 3C are block diagrams illustrating different messages exchanged between the base stations in accordance with some embodiments of the present application.

DESCRIPTION OF EMBODIMENTS

FIG. 1A depicts the deployment of multiple base stations with different types of coverage in accordance with some embodiments of the present application. As shown in the figure, there are four types of base stations, among which the macro eNB 10 has the largest service coverage, the pico eNBs 20 have smaller service coverage and the home eNBs 30 has the smallest service coverage. Sometimes, the macro eNB 10 is also referred to as a "macro cell" for creating a relatively large area of coverage and the other types of eNBs are commonly referred to as "low power nodes (LPN)." These low power base station nodes include remote radio heads, pico eNBs 20, home eNBs (HeNBs) 30, and similar components. Although having a smaller area of coverage than a macro cell, these LPNs are often deployed within an area covered by a macro cell to improve the quality and reliability of the service UE received in this area. The area of coverage created by a low power base station might be referred to as a pico cell, a femto cell, a hotzone cell, a small cell, or a similar term. In this application, the term "small cell" is used to refer the coverage created by those LPNs. The term "low power node" or "small cell eNB" are used interchangeably herein.
As shown in FIG. 1A, the heterogeneous network deployment consist of low power nodes being placed throughout a macro-cell layout. But, it is also possible to deploy standalone low power nodes without macro base stations. One difference between low power nodes and macro eNBs is the transmit power. For example, the maximum allowable transmit power of a macro eNB, a pico eNB, and a HeNB, are typically 46 dBm, 30 dBm, 20 dBm, respectively, for a 10 MHz carrier bandwidth.
Another difference between different types of low power nodes is the connection to the core network (i.e., backhaul) and the connection to neighbor cells. FIG. 1B depicts the communication interfaces between multiple base stations and mobile management entity (MME) servers in accordance with some embodiments of the present application. As shown in the figure, an eNB (50-1, 50-2, 50-3) may have a dedicated backhaul connection to the service provider's core network (e.g., the MME/S-GW servers 60-1 and 60-2) through the S1 interface defined in 3GPP and connections to other eNBs through the X2 interface defined in 3GPP). In some embodiments, a femto cell is a small cellular base station, typically designed for use in a home or small business. It connects to the service provider's core network via broadband (such as DSL or cable). As such, the HeNBs 30 in FIG. 1A may not have a direct interface like the X2 interface to other pico eNBs 20 or the macro eNB 10.
As noted above, energy consumption has become a more and more important issue for network operators due to a large number of base stations in the cellular networks. Although people have developed many schemes to find the minimal transmission power level that ensures the service coverage, quality, and capacity, these schemes by themselves are not sufficient to reduce the overall energy consumption of wireless networks because a large part of energy consumption remains consumed even at a low output transmit power level. This is partly due to the energy consumption by those load-independent components as well as the presence of control channels and common reference signals.
Recently, the heterogeneous network design of having multiple small cells deployed within a macro cell is becoming a very attractive solution, especially for providing better user experience in the outdoor/indoor high-traffic areas. An important nature of the heterogeneous network design is that the network traffic may be off-loaded from a macro eNB to a LPN or from one LPN to another LPN. For example, in a densely-populated urban region (e.g., a large shopping mall, etc.), many small cell low power nodes are deployed to support a large amount of data traffic.
On the other hand, the traffic load at any base station (macro or micro) may go up and down during a predefined time period (e.g., a day). Considering the traffic load fluctuation, it is possible to switch off a small cell base station when the traffic load is light. For example, when there is no UE in a small cell, or there is no active UE or connected UE in the small cell, or all the UE attached to the small cell are in idle mode or sleep mode, etc., the small cell base station may turn off its RF receiving/transmit circuitry completely. Alternatively, the small cell may turn off its RF transmit circuitry while keeping its RF receiving circuitry on, or make itself available or active in limited resources such as time and/or frequency in the air interface.
By implementing the idle/sleep mode, a network operator can reduce the energy consumption when a small cell base station enters the idle or sleep mode by turning off its transmit and/or receiving circuitry. Another benefit from this adaptive power consumption scheme is that it can reduce the interference to other macro and/or small cells especially in a densely-populated region. However, there are a number of issues that need to be addressed in order to support the sleep mode of a base station (which may be a macro eNB or a LPN). Exemplary issues include when the base station should enter the sleep mode, when and how to wake up a base station in the sleep mode, how to handle the UE connected to a base station that is going to enter the sleep mode, etc.
In general, a base station should enter the sleep mode when its traffic load is lower than a predefined level, which may adversely affect the coverage provision and/or traffic balance in the neighboring cells. Therefore, other base stations should be notified of a base station's sleep mode parameters (e.g., the start time and time duration, etc.) so that they can coordinate with each other to reduce the negative impact on the coverage provision.
Exemplary embodiments of the present invention will be described below in the context of a wireless communication system as shown in FIGS. 1A and 1B. As noted above, a wireless network is usually composed of a plurality of macro base stations in a cellular deployment. Within the coverage area of each macro base station, a set of small cells is deployed, forming a second layer, operated by the same operator. Very often, the small cells are deployed at hotspots of large capacity for offloading traffic from a corresponding macro base station. The small cells may be connected to the macro base station via an interface (e.g., X2 interface in the case of the operator-deployed open access small cells, or S1 interface in the case of the user-deployed closed access small cells.
In the present application, an exemplary sleep/wakeup mechanism is described as follows: when a small cell is not highly loaded and a neighboring macro base station or another small cell can handle the traffic from/to the small cell while offering users satisfactory QoS, the small cell should enter sleep mode. As the traffic load at the macro base station or the other small cell increases, one or more small cells need to be switched on depending on the traffic load and localization of traffic. In some embodiments, the base station that enters the sleep mode may not transmit any signal to UE in the air interface. In some embodiments, the base station may still transmit limited signals such as Synchronization CHannel (SCH) (or preambles) or Broadcast CHannel (BCH) (or some essential system information blocks) to UE, but it transmits fewer signals than it does during the active mode. On the other hand, the base station may still communicate with other base stations and the core network via a backhaul during the sleep mode.
FIGS. 2A and 2B are flow charts illustrating how an operation mode controller 101 coordinates the operation of multiple base stations (103-1, 103-3), sleep/wakeup mode, in a centralized way in accordance with some embodiments of the present application. In some embodiments, the function of sleep/wakeup control is performed by a new network entity, or a virtual entity in the network, which can be either implemented in the gateway or distributed in the base stations. In this example, this function is implemented in the existing network entities such as an MME server or a macro base station whose coverage may overlap with that of one or more small cells. The operation model controller 101 makes decisions about when a base station should enter or leave sleep mode based on some information such as the traffic load and user localization. In other words, the information of the positions of UE in a particular macro/small cell is available to the controller if the controller is implemented in the MME server where UE location information is available for mobility management purpose. Alternatively, the UE location information may also be available if UE reports its location information using the GPS equipment in the UE. Sometimes, the UE location information can be estimated by network positioning techniques. Note that in the following description, it is assumed the UE location information (relative or absolute, rough or accurate) is available and the details of how to obtain this location information is well-known to those skilled in the art and not within the scope of the present application.
In order to effectively control one or many small cells to enter and leave sleep mode, the operation mode controller 101 needs to know the traffic load and the UE service requirement of a particular small cell. In the current 3GPP LTE specifications, the traffic load in terms of physical resource block (PRB) utilization may be exchanged on the X2 interface between different cells. Note that such traffic load information itself is not enough for the operation mode controller 101 because that the PRB utilization information only reveals the resource usage in the frequency domain and it is a cell-level statistics that combines the statistics for all UE within the cell. To overcome this problem, a base station may need to report the resource usage per UE as well as the combined cell-level resource usage when requested by the operation mode controller 101.

As shown in FIG. 2A, the operation mode controller 101 sends resource usage requests 110-1 and 110-3 to the respective base stations, eNB1 103-1 and eNB 2 103-3. In some embodiments, the operation mode controller 101 sends the resource usage status request to a base station either periodically or on-demand, which may be transmitted on the X2 interface. The structure of an exemplary resource usage status request 200 is shown in FIG. 2C. In some embodiments, the resource usage status request 200 is a standalone message transmitted on the X2 interface. In some other embodiments, the request may be combined with other information into one message. Below is a table explaining the meaning of attributes in the resource usage status request 200 shown in FIG. 2C:

Attribute	Meaning
Cell ID
210	An identifier of a target cell that receives this request message
All UE in cell (1/0) 215	'1' means that the base station should report all UE usage information of this target cell; '0' means that the base station should report only resource usage information by the identified UE
List of UE IDs to report 220	If the "All UE in cell" attribute is '0', the operation mode controller should provide a list of UE IDs whose usage information need to be reported by the base station
Measurement Time Interval 225	The length of a period during which the resource usage for UE in the target cell is measured. For example, the value of this attribute may be defined as a number of sub frames.
Report period 230	The period for the base station to submit a resource usage information report. Note that '0' means the report is once while other values indicate the report is periodical.

Upon receipt of the resource usage status request, the two base stations, eNB 1 and eNB2, each respond with a resource usage information report 115-1 and 115-3, respectively. In some embodiments, the resource usage information is in the time domain. An example of the resource usage information report 240 from a base station to the operation mode controller is shown in FIG. 2D. In some embodiments, the resource usage information report 240 is a standalone message transmitted on the X2 interface. In some other embodiments, the report may be combined with other information into one message. Below is a table explaining the meaning of attributes in the resource usage information report 240 shown in FIG. 2D:

Attribute	Meaning
Cell ID
245	An identifier of a target cell that submits this report message.
All UE in cell (1/0) 250	'1' means that the report includes all UE usage information in the cell. This value corresponds to the same one contained in the resource usage status request 200.
Resource usage information 255	This attribute includes a plurality of pairs of (UE ID, usage string) 260-1 and 260-2, etc. For each UE ID, the measured UE usage information is defined as a usage string, wherein each position in the usage string represents a DL subframe, for which a non-zero value indicates the amount of resource utilized by that UE. The length of the usage string is defined by the "Measurement Time Interval" attribute in the resource usage status request message. In some embodiments, if the "All UE in cell" attribute is '0', this attribute only contains a plurality of usage strings, one for each UE ID in the UE list in the resource usage status request message.

In some embodiments, the usage string is a binary string such that a value "1" in the string indicates that a corresponding subframe is used by that UE and a value "0" in the string indicates otherwise. In some other embodiments, the UE resource usage information is defined as a non-binary string, whose value represents the percentage of the amount of resources used by that UE in a subframe. For example, the numerator of the percentage calculation corresponds to the number of physical resource blocks within the subframe used by the UE while the denominator of the percentage calculation is the total number of physical resource blocks in that subframe.
The operation mode controller 101 then analyzes the resource usage information reports coming from different base stations (120) and determines if they satisfy a predefined condition (125). For instance, if a report from one eNB indicates no active UE in a cell (e.g. all "0" in the list of UE resource usage information entry), the operation mode controller 101 may consider that cell to be a candidate cell for entering sleep mode. In another example, if a report from one eNB indicates low traffic load in a cell (e.g. very few "1" in the list of UE resource usage information entry) and its neighboring cells are not over loaded, the operation mode controller 101 may consider that cell to be a candidate for entering sleep mode. In some embodiments, a threshold is defined to determine whether the traffic load at a particular cell is low or not. Such threshold may be predefined and stored in the operation mode controller or dynamically calculated by the operation mode controller.

If the predefined condition is not met (125-No), the operation mode controller 101 may end this attempt of causing any base station to enter sleep mode. In some embodiments, the operation mode controller 101 may restart the process at a later time by resending resource usage status requests to the base stations. In some embodiments, the base stations periodically submits their resource usage information reports to the operation mode controller 101 so that the controller can reevaluate the usage status at different base stations. Assuming that the predefined condition is met (125-Yes), the operation mode controller 101 identifies the cell that can enter the sleep mode and sends a sleep command 135-1 to the base station eNB 1 associated with the cell. In addition, the operation mode controller 101 sends a sleep notification 135-3 to neighboring base stations including the base station eNB 2, alerting that the base station eNB 1 will enter the sleep mode. An example of the sleep/wakeup command 265 from the operation mode controller to a base station is shown in FIG. 2E. In some embodiments, the sleep/wakeup command 265 is a standalone message transmitted on the X2 interface. In some other embodiments, the command may be combined with other information into one message. Below is a table explaining the meaning of attributes in the sleep/wakeup command 265 shown in FIG. 2E:

Attribute	Meaning
Cell ID
270	An identifier of a target cell that receives this command message.
Sleep or Wake Up (1/0) 275	"1" means the target cell will enter sleep mode; "0" means the target cell will stay active or leave sleep mode after receiving this message.
List of UE for handover 280	If the 'Sleep or Wake up' entry is "1", the base station at the target cell needs to handover those active UE to neighboring cells in order to enter sleep mode. In this case, this entry includes a list of pairs of (UE ID, cell ID), the cell ID representing a corresponding cell to which the UE will be handed over.
List of UE for handover 280	If the 'Sleep or Wake up' entry is "0", the target cell should wake up and expect some UE handed over from neighboring cells. In this case, this entity includes a list of pairs of (UE ID, cell ID), the cell ID representing a corresponding cell from which the UE will be handed over.

As shown in FIG. 2A, after completing the UE handover (140), the base station eNB 1 may enter sleep mode (145) if there's no active UE left in the cell. If the base station eNB 2 does not have the capacity of handling all the UE from the base station eNB 1, the operation mode controller 101 may need to find another base station for receiving the remaining UE handed over from the base station eNB 1. Note the details of UE handover procedures and the corresponding X2 messages are known in the art.
FIG. 2B illustrates a process of waking up a base station in the sleep mode in accordance with some embodiments of the present application. Before waking up a small cell in the sleep mode, the operation mode controller makes some predictions of the load information. Such load information can be, for instance, derived from localization methods, based on localization algorithms such as by exploiting traffic information in neighboring sites of that sleep mode cell. In this example, the operation mode controller 101 sends a resource usage status request 150 to a base station and receives a resource usage information report 155 from the base station. Note that the operation mode controller 101 may communicate with multiple base stations to learn their traffic load and analyzes the reports 160 to determine whether a predefined condition is met or not (165). For example, when the resource usage information reports from several neighboring cells of one sleep mode cell indicate an overall high traffic load (e.g. no "0" in the list of UE usage information entries) in that particular area, the operation mode controller 101 may consider waking up the cell in the sleep mode by sending a wake up command 175-1 to offload the traffic and/or to provide QoS to UEs, which otherwise cannot be met. A threshold can be defined to determine whether the traffic load is high or not. Such threshold may be predefined and stored in the operation mode controller or dynamically calculated by the operation mode controller. In response to the wakeup command, the base station eNB 1 enters the work mode 180. In some embodiments, the operation mode controller 101 also sends a wake notification 175-3 to a neighboring base station eNB 2, the notification instructing the base station to start the handover procedure 185 for some of the UE so as to take away some traffic load from the base station.

FIG. 3A is a flow chart illustrating how one base station coordinates its operation with multiple base stations in accordance with some embodiments of the present application. In this example, the function of sleep/wake up mode is performed in a distributed way and there is no centralized operation mode controller. As shown in the figure, the base station eNB 1 103-1 has two neighboring base stations, eNB 2 103-3 and eNB 3 103-5. The base station eNB 1 first checks whether a predefined condition is met or not (305). In some embodiments, the base station periodically checks whether its traffic load is below a threshold (e.g., a predefined fixed or dynamic calculated load parameter). If the condition is met (305-Yes), this base station sends a sleep mode request message 310 to its neighboring sites (including the macro cell) such as the base station eNB 2. An example of a sleep mode request 400 from a base station is shown in FIG. 3B. In some embodiments, the sleep mode request 400 is a standalone message transmitted on the X2 interface. In some other embodiments, the sleep mode request 400 may be combined with other information into one message. Below is a table explaining the meaning of attributes in the sleep mode request 400 shown in FIG. 3B:

Attribute	Meaning
Source cell ID 405	An identifier of a source cell that sends this request message.
Target cell ID 410	An identifier of a target cell that received this request message.
Sleep start time 415 and sleep end time 420	The proposed start and end time for the source cell to enter sleep mode. For example, the time can be defined as the SFN number.
List of UE for handover 425	A list of UE that the source cell wants to hand over to the target cell in order to enter sleep mode. In some embodiments, this entry includes a plurality of pairs of (UE ID, usage string) 430-1 and 430-2. For each UE ID, each position in the usage string represents a DL subframe, for which a non-zero value indicates the amount of resource utilized by that UE. The usage string is a UE resource usage history in the source cell. The length of the usage string may be 10 or longer.

Based on its own traffic load, a base station that receives this sleep mode request determines the amount of resource it has to accommodate the list of UE IDs in the sleep mode request from the source cell and then replies an acceptance in a sleep mode response 315 to the source cell. Note that a base station that has a high traffic load and/or expect more UE may enter its service coverage may decline the request in the sleep mode response message. An example of a sleep mode response 440 from a base station is shown in FIG. 3C. In some embodiments, the sleep mode response 440 is a standalone message transmitted on the X2 interface. In some other embodiments, the sleep mode response 440 may be combined with other information into one message. Below is a table explaining the meaning of attributes in the sleep mode response 440 shown in FIG. 3C:

Attribute	Meaning
Source cell ID 445	An identifier of a source cell that sends the request message 400.
Target cell ID 450	An identifier of a target cell that received the request message 400.
Acceptance or decline (1/0) 455	"1" means the base station associated with the "target cell ID" accepts a UE handover from the base station associated with the "source cell ID" that wants to enter sleep mode; "0" means the base station associated with the "target cell ID" declines the UE handover from the base station associated with the "source cell ID" that wants to enter sleep mode.
List of UEs for handover 460	A list of UE IDs (465-1, 465-2, ...) associated with the based station identified by the "source cell ID" attribute that the base station associated with the "target cell ID" attribute can accommodate. For example, if the 'Acceptance or decline' is "1", then this UE list contains the same list of UE IDs in the original "sleep mode request" message. Sometimes the list of UEs for handover may be empty in this case. Otherwise, if the 'Acceptance or decline' is "0", then this UE list may contain a subset of the original UE ID list in the "sleep mode request" message that the base station can accommodate or empty.
Sleep start time 470 and sleep end time 475	The start and end time for the source cell identified by the "source cell ID" attribute to enter sleep mode, which correspond to the two attributes 415 and 420, respectively.

After analyzing the sleep mode response message (320), the source cell base station eNB 1 may start the UE handover procedure 325 for the UE listed in the non-empty "List of UE for handover" attribute in the sleep mode response 315 with its neighboring cells. Again, the details of UE handover procedures and the corresponding X2 messages are known in the art. Note that the UE handover procedure 325 happens only when the "List of UEs for handover" attribute 460 in the sleep mode response 315 is not empty .
If there is still active UE left at the source cell (335-yes) after the handover with the base station eNB 2, the base station eNB 1 may start a new round of a sleep mode request 345, a sleep mode response 350, an analysis of the sleep mode response message 352, and a handover procedure 355 with another neighboring cell (e.g., base station eNB 3). In other words, the base station eNB 1 103-1 iteratively negotiates with its neighboring cells to hand over all the UE to the neighboring cells. This could happen after several rounds of decline response and partial UE handover in each round with the last round sleep response being acceptance. Only after there is no active UE in the source cell at the base station eNB 1 and it receives no decline in the sleep mode response message from the last neighboring cell to which it sends its sleep mode request, the base station eNB 1 may enter the sleep mode (340) at the start time and then leaves sleep mode at the end time, both of which are in the sleep mode request.
While particular embodiments are described above, it will be understood it is not intended to limit the invention to these particular embodiments. On the contrary, the invention includes alternatives, modifications and equivalents that are within the spirit and scope of the appended claims. Numerous specific details are set forth in order to provide a thorough understanding of the subject matter presented herein. But it will be apparent to one of ordinary skill in the art that the subject matter may be practiced without these specific details. In other instances, well-known methods, procedures, components, and circuits have not been described in detail so as not to unnecessarily obscure aspects of the embodiments.
Although the terms first, second, etc. may be used herein to describe various elements, these elements should not be limited by these terms. These terms are only used to distinguish one element from another. For example, first ranking criteria could be termed second ranking criteria, and, similarly, second ranking criteria could be termed first ranking criteria, without departing from the scope of the present invention. First ranking criteria and second ranking criteria are both ranking criteria, but they are not the same ranking criteria.
The terminology used in the description of the invention herein is for the purpose of describing particular embodiments only and is not intended to be limiting of the invention. As used in the description of the invention and the appended claims, the singular forms "a," "an," and "the" are intended to include the plural forms as well, unless the context clearly indicates otherwise. It will also be understood that the term "and/or" as used herein refers to and encompasses any and all possible combinations of one or more of the associated listed items. It will be further understood that the terms "includes," "including," "comprises," and/or "comprising," when used in this specification, specify the presence of stated features, operations, elements, and/or components, but do not preclude the presence or addition of one or more other features, operations, elements, components, and/or groups thereof.
As used herein, the term "if' may be construed to mean "when" or "upon" or "in response to determining" or "in accordance with a determination" or "in response to detecting," that a stated condition precedent is true, depending on the context. Similarly, the phrase "if it is determined [that a stated condition precedent is true]" or "if [a stated condition precedent is true]" or "when [a stated condition precedent is true]" may be construed to mean "upon determining" or "in response to determining" or "in accordance with a determination" or "upon detecting" or "in response to detecting" that the stated condition precedent is true, depending on the context.
Although some of the various drawings illustrate a number oflogical stages in a particular order, stages that are not order dependent may be reordered and other stages may be combined or broken out. While some reordering or other groupings are specifically mentioned, others will be obvious to those of ordinary skill in the art and so do not present an exhaustive list of alternatives. Moreover, it should be recognized that the stages could be implemented in hardware, firmware, software or any combination thereof.

Claims

(Currently Amended) A method, performed by a first base station (103-1), for controlling the operation mode of the first base station with multiple base stations (103-3, 103-5), comprising:
the first base station (103-1) measuring resource usage for UE associated with the first base station;

when the resource usage at the first base station is below a predefined threshold:
the first base station (103-1) performing a sleep mode negotiation including the following steps a) to c):
a) sending a sleep mode request (400) to a second base station (103-3), the request including an identifier of the first base station, an identifier of the second base station, a proposed sleeping interval, UE identity information and associated UE resource usage information of the UE;

b) receiving a sleep mode response (440) from the second base station (103-3), the response including the identifier of the first base station, the identifier of the second base station, an acceptance indicator, the UE identity information and sleeping interval information corresponding to that in the sleep mode request;

c) starting a handover process with the second base station (103-3) in accordance with the UE identity information in the sleep mode response; and

the first base station (103-1) further entering a sleep mode in response to determining that no UE is left in the first base station after a completion of the handover process and in response to determining by the first base station that the sleep mode response does not include a decline indication to decline the handover process; and

when there is still active UE left at the first base station after a completion of the handover process with the second base station, the first base station (103-1) performing a new round of said sleep mode negotiation with a third base station (103-5) to handover the still active UE at the first base station.
The method of claim 1, wherein the sleep mode request includes a plurality of pairs of "UE ID, usage string", the usage string including a plurality of values indicating the resource usage of the UE associated with the UE ID within a resource usage measurement time interval.
The method of claim 2, wherein the usage string is a binary string and a non-zero value in the binary string represents that the UE uses resources during a subframe corresponding to a position of the non-zero value in the binary string.
The method of claim 2, wherein the usage string is a non-binary string and a non-zero value corresponding to a position in the non-binary string represents a percentage of physical resource blocks used by the UE during a corresponding subframe.
The method of any of claims 1 to 4, wherein the UE identity information in the sleep mode response is at least a subset of the UE identity information in the sleep mode request.